The Toe to Heel Waterflooding (TTHW) process is described in U.S. Pat. No. 6,167,966, issued Jan. 2, 2001 to the same assignee as the present case. Briefly, the TTHW consists of guiding the advance of a liquid displacement front originating from an injection well by having a production well with an open horizontal leg oriented towards the injection well act as a linear pressure sink to which the front is attracted and by which the front is guided. The present invention is directed to the problem of watering out or coning associated with continued production during the TTHW process, after initial production occurs at the “toe” of the horizontal leg of the production well.
Irrespective of whether premature water break-through in a horizontal well is coming from a coning situation in a reservoir with bottom water, or from a waterflood operation, the zonal isolation and blocking of a portion of the reservoir through which water is coning is a very complex and costly operation which generally involves the following steps or operations. Firstly, identifying the “culprit/offending” zone and secondly, isolating the zone by some kind of blockage. The identification of the “offending zone” is usually made with a production logging operation.
For both heavy and light oil reservoirs with large thickness and high permeability, or even for low permeability reservoirs (if they have a streak of high permeability at the bottom or if horizontal permeability increases downwards), the TTHW process, which in the field takes the name of “water injection at the toe”, seems to be very efficient, and is currently undergoing field testing. For intermediate and heavy oil reservoirs, the application of TTHW is almost a requirement, as it entails a short-distance oil displacement, as compared to the long-distance displacement in the conventional waterflooding. Conventional waterflooding in heavy oil reservoirs is associated with either very large pressure gradients or premature water break-through, and both these aspects lead to low injectivity or poor sweep efficiency, and result in poor oil recovery. In the scenarios mentioned above TTHW is a process leading to a better sweep efficiency and hence higher oil recovery.
The TTHW process disclosed in U.S. Pat. No. 6,167,966 involves providing one or more water injection wells completed low in the reservoir, and one or more production well having a horizontal leg completed high in the reservoir. The horizontal leg is oriented toward the injection well, with its toe close to the injection well. In a preferred embodiment, water injection is started at the injection well(s) and a laterally extending, quasi-upright waterflood front is advanced toward the horizontal production well. The production well is kept open and continuously produces oil, creating a linear, low pressure sink. The sink acts to attract and guide the advance of the laterally extending front along its length. It has been found that the waterflood front will stay quasi-upright and its direction of advance is controlled to yield good vertical and lateral sweep. This embodiment is referred as the single-stage version of the TTHW process.